A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to but distinguishable from tornadoes which generally have convergent damage.

History of term

The term was defined by severe weather expert Tetsuya Theodore Fujita as affecting an area 4 km (2.5 mi) in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Dr. Fujita also coined the term macroburst for downbursts larger than 4 km (2.5 mi).

A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga. They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst).

Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).

Dry microbursts

When rain falls below cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst.

Dry microbursts, produced by high based thunderstorms that generate little surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. (Wakimoto, 1985) developed a conceptual model (over the High Plains) of a dry microburst environment that comprised of three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, and low surface relative humidity.

Wet microbursts

Wet microbursts are downbursts accompanied by significant precipitation at the surface (Fujita, 1985) which are warmer than their environment (Wakimoto, 1998). These downbursts rely more on the drag of precipitation for downward acceleration of parcels than negative buoyancy which tend to drive "dry" microbursts. As a result, higher mixing ratios are necessary for these downbursts to form (hence the name "wet" microbursts). Melting of ice, particularly hail, appears to play an important role in downburst formation (Wakimoto and Bringi, 1988), especially in the lowest one kilometer above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts a difficult task.

Characteristic	Dry Microburst	Wet Microburst
Location of Highest Probability	Midwest/West	Southeast
Precipitation	Little or none	Moderate or heavy
Cloud Bases	As high as 500 mb	Usually below 850 mb
Features below Cloud Base	Virga	Shafts of strong precipitation reaching the ground
Primary Catalyst	Evaporative cooling	Downward transport of higher momentum
'''Environment below Cloud Base	Deep dry layer/low relative humidity/dry adiabatic lapse rate	Shallow dry layer/high relative humidity/moist adiabatic lapse rate
Surface Outflow Pattern	Omni-directional	Gusts of the direction of the mid-level wind

Development stages of microbursts

The University of Illinois breaks the evolution of downbursts into three stages, the contact stage, the outburst stage and the cushion stage.

<gallery style="font-size: small;"> Image:contactstage.jpg|A downburst initially develops as the downdraft begins its descent from cloud base. The downdraft accelerates and within minutes, reaches the ground (contact stage). It is during the contact stage that the highest winds are observed. Image:outburststage.jpg|During the outburst stage, the wind "curls" as the cold air of the downburst moves away from the point of impact with the ground. Image:cushionstage.jpg|During the cushion stage, winds about the curl continue to accelerate, while the winds at the surface slow due to friction. </gallery>

Physical processes of dry and wet microbursts

Simple explanation

In the case of a wet microburst, the atmosphere is warm and humid in the lower levels and dry aloft. As a result, when thunderstorms develop, heavy rain is produced but some of the rain evaporates in the drier air aloft. As a result the air aloft is cooled thereby causing it to sink and spread out rapidly as it hits the ground. The result can be both strong damaging winds and heavy rainfall occurring in the same area. Wet downbursts can be identified visually by such features as a shelf cloud, while on radar they sometimes produce bow echoes.

In the case of a dry microburst, the atmosphere is warm but dry in the lower levels and moist aloft. Thus when showers and thunderstorms develop, most of the rain evaporates before reaching the ground.

Basic physical processes using simplified buoyancy equations

Start by using the vertical momentum equation



By decomposing the variables into a basic state and a perturbation, defining the basic states, and using the Ideal Gas Law (), then the equation can be written in the form



where B is used to denote buoyancy. Note that the virtual temperature correction usually is rather small and to a good approximation, it can be ignored when computing buoyancy. Finally, the effects of precipitation loading on the vertical motion are parameterized by including a term that decreases buoyancy as the liquid water mixing ratio () increases, leading to the final form of the parcel's momentum equation:



The first term is the effect of perturbation pressure gradients on vertical motion. In some storms this term has a large effect on updrafts (Rotunno and Klemp, 1982) but there is not much reason to believe it has much of an impact on downdrafts (at least to a first approximation) and therefore will be ignored.

The second term is the effect of buoyancy on vertical motion. Cleary, in the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment. This cooling typically takes place as a result of phase changes (evaporation, melting, and sublimation). Precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy. The major contribution to this process is from evaporation.

The last term is the effect of water loading. Whereas evaporation is promoted by large numbers of small droplets, it only takes a few large drops to contribute substantially to the downward acceleration of air parcels. This term is associated with storms having high precipitation rates. Comparing the effects of water loading to those associated with buoyance, if a parcel has a liguid water mixing ration of 1.0 gkg^-1, this is roughly equivalent to about 0.3 K of negative buoyancy; the latter is a large (but not extreme) value. Therefore, in general terms, negative buoyancy is typically the major contributor to downdrafts.

Negative vertical motion associated only with buoyancy

Using pure "parcel theory" results in a prediction of the maximum downdraft of



where NAPE is the Negative Available Potential Energy,



and where LFS denotes the Level of Free Sink for a descending parcel and SFC denotes the surface. This means that the maximum downward motion is associated with the integrated negative buoyancy. Even a relatively modest negative buoyancy can result in a substantial downdraft if it is maintained over a relatively large depth. A downward speed of 25 m/s results from the relatively modest NAPE value of 312.5 m²s^-2. To a first approximation, the maximum gust is roughly equal to the maximum downdraft speed.

See the following reference and link for more information on derivation of buoyancy equations:

• Extreme Convective Windstorms: Current Understanding and Research

Danger to aircraft

Further information: Downburst

The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and landing. The following are some fatal crashes that have been attributed to microbursts in the vicinity of airports:

• Eastern Airlines Flight 66, John F. Kennedy International Airport - June 24, 1975
• Pan Am Flight 759, Miami International Airport - July 9, 1982
• Delta Airlines Flight 191, Dallas-Fort Worth International Airport - August 2, 1985
• Martinair Flight 495, Faro Airport - December 21, 1992
• USAir Flight 1016, Charlotte/Douglas International Airport - July 2, 1994
• Goodyear Blimp, Coral Springs, Florida - June 16, 2005
• Air France Flight 358, Toronto Pearson International Airport - August 2, 2005
• One-Two-GO Airlines Flight 269, Phuket International Airport - September 16, 2007

A microburst often causes aircraft to crash when they are attempting to land (except Pan Am Flight). The microburst is an extremely powerful gust of air that, once hitting the ground, spreads in all directions. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed. When the microburst hits, the pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst. A pilot inexperienced with microbursts would try to decrease the speed. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings. The sudden loss of air moving across the wings causes the aircraft to literally drop out of the air. The best way to deal with a microburst in an aircraft would be to increase speed as soon as the spike in airspeed is noticed. This will allow the aircraft to remain in the air when traveling through the tailwind portion of the microburst and also pass through the microburst with less difficulty, although it is possible that for light aircraft, the descent rate induced by the microburst will exceed their maximum climb rate, leading to an unavoidable crash.

Effects of microbursts

A microburst often has high winds that can knock over full grown trees. They usually last for a couple of seconds.

List of notable microbursts

• August 14, 1996 - Runyan calls this the costliest storm in Arizona history. A severe thunderstorm and its accompanying dry microburst hit the northwest portion of the Phoenix metro area – ripping off tile roofs and causing $160 million in damage. An Arizona record wind gust of 115 miles per hour is recorded at the Deer Valley Airport. A few locations had to go without power for several days.
• A microburst squall with windspeeds of 80 miles per hour is responsible for capsizing and sinking the Pride of Baltimore in May 1986 in the Caribbean, about 250 miles north of Puerto Rico. The ship took the lives of her captain and three of her other 11 crew members.
• A particularly violent microburst is a possible alternative explanation to the 1961 sinking of the American school brigantine Albatross. The ship's captain Dr. Christopher Sheldon claimed that the ship was hit by a white squall on the voyage from Progreso, Yucatán, to Nassau in the Bahamas.
• A microburst cost the New Jersey suburban towns of Bloomfield, Cedar Grove Montclair, and Verona a combined total of a little less than $1 million in damages when a storm passed through the area on July 18 2006^[1][2]
• A microburst moved through northern and western Utah during the late afternoon and evening hours on June 5, 1995. Some of the higher reported wind gusts were: Tremonton 95 mph, Highland and American Fork 90 mph, Pleasant Grove 88 mph, and north Orem 86 mph. According to data received from the Western Insurance Information Service, damage estimates totaled $15 million.
• In September 1998, a microburst hit the city of Syracuse, New York. Syracuse University was closed for this first time in over a decade because of the destruction.
• On March 12, 2006 a severe microburst with winds varying from 70 to 90 mph damaged large portions of Lawrence, Kansas. Reported damage included downed power lines, stop lights and trees, overturned semi-trailers, collapsed farm silos and damage to roofs. Seventy buildings on the University of Kansas campus reported damage. In total, over $8 million in damages was estimated. ^[3]
• On August 9, 2007 a microburst with winds of 85 mph affected the city of Salem, Ohio. The roofs of two gas stations were damaged. One of the gas stations, which had only been open for 5 weeks, had portions of its roof torn off, which subsequently damaged cars as it was blown down the street. Windows in cars and at the Giant Eagle grocery store were broken. Many other businesses reported damage. ^[4]
• On September 9, 2007 a severe microburst hit the Massachusetts College of Liberal Arts (MCLA) in North Adams, Massachusetts. Many trees came down and the roofs of several town-house style dorms were damaged. The National Weather Service in Albany, New York estimated that there were straight-line winds with gusts of 110 miles per hour. There were no injuries reported. ^[5]

Printed media

• Fujita, T.T. (1981). "Tornadoes and Downbursts in the Context of Generalized Planetary Scales". Journal of the Atmospheric Sciences, 38 (8).
• Fujita, T.T. (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.
• Wilson, James W. and Roger M. Wakimoto (2001). "The Discovery of the Downburst - TT Fujita's Contribution". Bulletin of the American Meteorological Society, 82 (1).

References

See also

• NEXRAD
• Air safety
• Downdraft
• Vertical draft
• Wind shear
• Low level windshear alert system (LLWAS)
• Weather radar
• Planetary boundary layer (PBL)

External links

• Arizona's Worst 5 Monsoon Storms, as of 2005
• The Semi-official Microburst Handbook Homepage (NOAA)
• Microbursts (WW2010) (University of Illinois at Urbana-Champaign)
• Taming the Microburst Windshear (NASA)
• Microbursts {University of Wyoming)
• Forecasting Microbursts & Downbursts (Forecast Systems Laboratory)